Sensing devices are widely used in consumer, commercial, industrial, medical, environmental, and military applications. Sensors that detect molecules, that is biosensors and chemical sensors of various types, are gaining increasing importance in the sensor field. Sensors currently exist that detect volatile organic compounds in the air and can be used to differentiate odors. Other types of sensors, such as immunosensors, can detect certain small or large molecules with a high degree of specificity. Still others detect very large molecules, such as DNA or RNA. There exist yet other sensors that are useful in detection of biochemical surface markers on disease-causing bacteria or viruses. For some types of sensors, low sensitivity may not be a limiting problem, as it may have been in the past. Instead, the sensitivity of a particular sensor may be so great that a relatively small number of molecules may be detected if they enter the operating zone (detection area or “sweet spot”) of the sensor.
Technologies are being developed with the potential to detect single molecules or individual molecular events. Such technologies may also be useful in detecting a single organism that may be harmful, such as: a bacterium, virus, or other pathogen, such as a prion. Detection of extremely low concentrations of toxic gas molecules in the air also may be achieved. However, if the concentration of the species to be detected (analyte) is very low, the probability of a single molecule entering the operating zone of the sensor would also be very low, and the arrival of analyte in a relatively tiny operating zone of a sensor might be a rare event. The probable time for the analyte in so low a concentration to migrate to the sensor operating zone could consequently be long or excessively long. Thus, the analyte would often not be detected quickly, if at all, even if the sensitivity of the sensor were more than adequate. For simplicity, analytes are often referred to herein as molecules or particles.
One solution to this problem of infrequent encounters between analyte and sensor is to increase the surface area of the sensor in contact with the milieu in which the analyte is to be detected. This approach has several distinct disadvantages. Typically, the cost of material of which sensors are made is high. Increasing the effective area of the operating zone may therefore be expensive. In addition, making the sensor much larger may often defeat its purpose. For example, it might no longer be sized appropriately for the application, as in an implantable device for human use or an unobtrusive detector for possible bioterrorism. Another solution to the problem is to concentrate the analyte prior to analysis. This approach may be less costly and may entail minimal increase in size of the overall system, however, the signal to noise ratio might not be improved if molecules in the milieu that interfere nonspecifically with detection of the analyte are concentrated along with the analyte.